Primordial neutrino asymmetry evolution with full mean-field effects and collisions. (arXiv:2110.11889v1 [hep-ph])
<a href="http://arxiv.org/find/hep-ph/1/au:+Froustey_J/0/1/0/all/0/1">Julien Froustey</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Pitrou_C/0/1/0/all/0/1">Cyril Pitrou</a>
Neutrino oscillations and mean-field effects considerably enrich the
phenomenology of neutrino evolution in the early Universe. Taking into account
these effects, most notably the neutrino self-interaction mean-field
contribution, we revisit the problem of the evolution of primordial neutrino
asymmetries including for the first time the complete expression for
collisions, which describe scattering and annihilations with electron/positrons
and reactions among (anti)neutrinos. We show that a generalisation of the
adiabatic transfer of averaged oscillations (ATAO) scheme, a numerical method
previously developed without neutrino degeneracy and based on the large
separation of time scales in this problem, is sufficient to reach the same
accuracy as the full quantum kinetic equation integration, but is notably
faster. This approximation highlights the physics of synchronous oscillations
at play in the evolution of neutrino chemical potentials, especially in the
particular case with only two-neutrino mixing. In particular, it allows to
understand what controls the beginning and the amplitude of oscillations, but
also why there is a subsequent regime of collective oscillations with larger
frequencies. We also find that it is very important to use the full collision
term instead of relying on damping-like approximations, in order not to
overestimate how collisions reduce these synchronous oscillations. Finally we
study qualitatively how mixing parameters affect the final neutrino
configuration, and in particular we show that the CP-violating Dirac phase
cannot substantially affect the final $N_{rm eff}$ nor the final electronic
(anti)-neutrino spectrum, and thus should not affect cosmological observables.
Neutrino oscillations and mean-field effects considerably enrich the
phenomenology of neutrino evolution in the early Universe. Taking into account
these effects, most notably the neutrino self-interaction mean-field
contribution, we revisit the problem of the evolution of primordial neutrino
asymmetries including for the first time the complete expression for
collisions, which describe scattering and annihilations with electron/positrons
and reactions among (anti)neutrinos. We show that a generalisation of the
adiabatic transfer of averaged oscillations (ATAO) scheme, a numerical method
previously developed without neutrino degeneracy and based on the large
separation of time scales in this problem, is sufficient to reach the same
accuracy as the full quantum kinetic equation integration, but is notably
faster. This approximation highlights the physics of synchronous oscillations
at play in the evolution of neutrino chemical potentials, especially in the
particular case with only two-neutrino mixing. In particular, it allows to
understand what controls the beginning and the amplitude of oscillations, but
also why there is a subsequent regime of collective oscillations with larger
frequencies. We also find that it is very important to use the full collision
term instead of relying on damping-like approximations, in order not to
overestimate how collisions reduce these synchronous oscillations. Finally we
study qualitatively how mixing parameters affect the final neutrino
configuration, and in particular we show that the CP-violating Dirac phase
cannot substantially affect the final $N_{rm eff}$ nor the final electronic
(anti)-neutrino spectrum, and thus should not affect cosmological observables.
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